This application claims the benefit of Korean Patent Application No. 1999-68051, filed on Dec. 31, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a method of driving an X-ray imaging device, and more particularly to a method of driving an X-ray imaging device that is capable of improving a picture""s contrast ratio and of shortening a driving time.
2. Discussion of the Related Art
Imaging systems that employ X-rays have been used in numerous medical, scientific and industrial applications. Such imaging systems include X-ray imaging devices. One type of X-ray imaging device uses an array of photosensitive cells on an array panel to sense the intensity of X-rays passing through an object. Those X-ray intensities are used to produce an image of the object. In operation, the photosensitive cells generate electric charges in proportion to the intensity of the X-rays. The electric charges from the photosensitive cells are applied to a signal converter that converts the charges into electrical signals, which are in turn sent to an image output device. The image output device processes the electrical signals so as to display the X-ray intensity pattern on a screen.
FIG. 1A and FIG. 1B respectively illustrate a sectional schematic view and a planar schematic view of a photosensitive cell in a photosensitive cell array panel. Referring to FIG. 1A, the photo-sensitive cell includes a gate line 22, a thin film transistor (TFT) 24, a charging capacitor (Cst) layer that are formed on a glass substrate 20, and a pixel electrode 32 that is connected to a drain electrode 26 of the TFT and to the charging capacitor Cst. The photo-sensitive cell further includes a gate electrode 30, a source electrode 28, a photo-sensing layer 34 on the pixel electrode 32, an insulating layer 36 on the photo-sensing layer 34, and an upper electrode 38.
The photo-sensing layer 34 is a photoconductive layer that is used for sensing X-rays and for converting those rays to electric charges. The photo-sensing layer 34 is beneficially formed from amorphous selenium having a thickness of hundreds of xcexcm.
As shown in FIG. 1A and FIG. 1B, the gate electrode 30 electrically connects to the gate line 22. Control signals are applied to the TFT by the gate line and by the gate electrode. The source electrode 28 connects to a data line 40 (see FIG. 1B). Beneficially, the data line 40 is perpendicular to the gate line 22. As previously indicated, the drain electrode 26 of the TFT 24 connects to the pixel electrode 32. As indicated by FIG. 1B, the pixel electrode 32 has an area that is as large as possible. This aids the collection of electric charges from the photo-sensing layer 34. The collected charges are then stored in the charging capacitor Cst. A high voltage generator 42 connects to the upper electrode 38. That voltage generator supplies a voltage that generates an electric field through the photo-sensing layer 34.
As X-rays irradiate an object, the rays that pass through the object are incident on the photo-sensing layer 34. Those incident X-rays produce electron-hole pairs in the photo-sensing layer 34. When a high voltage (several kilovolts) from the high voltage generator 42 is applied to the upper electrode 38, the electron-hole pairs within the photo-sensing layer 34 are separated by the resulting electric field. As shown in FIG. 1A, the holes are collected by the pixel electrode 32 and are stored in the charging capacitor Cst. Alternatively, electrons can be collected and stored. The TFT 24 acts as a switch that controls the discharge of electric charges in the charging capacitor Cst. When a gate voltage is applied to the gate electrode 30 via the gate line 22, a channel is defined between the source electrode 28 and the drain electrode 26. At this time, the electric charge in the charging capacitor Cst is applied to the source electrode 28 via the drain electrode 26. The electric charges applied to the source electrode 28 are then output over the data line 40, which is connected to the source electrode 28.
FIG. 2 illustrates an X-ray imaging system having a driving apparatus that converts electric charges stored in a photo sensitive cell array panel into electrical signals that can be output as an image. The X-ray imaging system includes a photo sensitive cell array panel 60 having a plurality of photo-sensing cells 62 that are arranged in a matrix. A gate driver 64 connects to gate lines, that gate lines GL1 to GLm, that are provided on the photo sensitive cell array panel 60. A data reader 66 connects to data lines, the data lines DL1 to DLn, that are also provided on the photo sensitive cell array panel 60. An output 68 displays the electrical signals from the data reader 66 as an image.
In the photo sensitive cell array panel 60 the photo-sensing cells 62 are individually positioned at intersections between the gate lines GL1 to GLm and the data lines DL1 to DLn. In FIG. 2, each of the photo-sensing cells 62 consists of a photo sensor 70, a charging capacitor Cst and a TFT 72. For each photo-sensing cell 62, a gate electrode 74 connects to the gate driver 64 by one of the gate lines GL1 to GLm, and a source electrode 76 connects to the data reader 66 by one of the data lines DL1 to DLn. Furthermore, a drain electrode 78 connects to a charging capacitor Cst.
When a gate control signal from the gate driver 64 is applied, via one of the gate lines GL1 to GLm, to the gate electrode 74 of the TFT 72 of a photo-sensing cell 62, a conductive channel is defined between the drain electrode 78 and the source electrode 76 of the TFT 72. Electric charges stored in the charging capacitor Cst are then transferred to the data reader 66, via one of the data lines DL1 to DLn, by the source electrode 76.
The gate driver 64 sequentially applies pulse-shaped gate control signals to the gate lines GL1 to GLm on the photo sensitive cell array panel 60. When a gate control signal is applied to one of the gate lines, the electric charges stored in the photo-sensing cells 62 connected to that gate line are applied to the data reader 66, thereby forming a scan line. The data reader 66 typically includes n charge amplifiers (not shown) connected to the n data lines DL1 to DLn. The charge amplifiers convert the flow of electric charges (current signals) from the data lines DL1 to DLn into voltage signals. Thus, the data reader 66 generates electrical data signals that correspond to electric charges from the photo sensitive cell array panel 60.
The data reader 66 sequentially applies the n electrical data signals, each of which depends on the intensity of the X-ray energy irradiated onto a photo sensitive cell, and a reference signal to the output 68. The output 68 includes a differential amplifier and an analog-to-digital converter (which are not shown). The electrical data signals input to the output 68 is an analog signal that includes noise. The output 68 differentially amplifies each electrical data signal and the reference signal to remove that noise, and then converts the noise-removed analog signal into a digital signal that is suitable for display on a screen as part of an image.
In an X-ray imaging device that operates as described above, the period of time that the high voltage is applied by the high voltage generator 42 to the upper electrode 38, and the period of time that X-rays are irradiated have a significant impact on the quality of the output image. An instantaneous current is generated at the photo-sensing layer 34 when the high voltage is first applied to the upper electrode 38. This accumulates a dark charge in the charging capacitor Cst. When the high voltage is removed, a variation in the charge stored in the charging capacitor Cst occurs, and thus a signal variation caused by this variation is generated. The image distortion problems caused by the dark charges and signal variations are results of a problem with the method of driving conventional X-ray imaging devices. That problem is described below in conjunction with FIG. 3 and FIG. 4.
FIG. 3 helps explain a method of driving a conventional X-ray imaging device. FIG. 3 shows a data process from the application of the high voltage until the read-out of the stored electric charges in the charging capacitor Cst. In the conventional driving method, a high voltage is applied by the high voltage generator 42 to the upper electrode 38 prior to irradiating an X-ray image onto the photosensitive cell array panel 60. Pixel charges are then stored in the charging capacitor Cst of each photo-sensing cell 62 as X-rays are irradiated. Then, the high voltage is turned off. Next, the electric charges stored in the charging capacitors Cst are sequentially read as scanning lines by the data reader 66. In such a driving method, charges are produced and stored both by X-ray irradiation and by dark charges that are produced by instantaneous currents created by turning-on the high.
FIG. 4 shows characteristic graphs of a time-dependent current variation at the photo-sensing layer and of a time-dependent variation in the electric charge that is stored in the charging capacitor Cst. As shown in FIG. 4, the high voltage applied to the upper electrode 38 prior to X-ray irradiation causes a current flow in the photo-sensing layer 34. This is most pronounced at time T0, which is the time that the high voltage is first applied. This causes a considerable amount of dark charges to be stored in the charging capacitor. When X-ray irradiation is performed, between times T1 and T2, a considerable amount of dark charge has been stored. The pixel charges produced by X-ray irradiation adds to the dark charges. Both charges are included in the total charges that are read from the charging capacitor Cst by the data reader 66 during data scanning, which occurs after the high voltage is turned-off. As a result, distortion problems and image quality deterioration can occur.
To solve such problems, the dark charge included in the total charge must be removed. To accomplish this, the conventional method turns the high voltage applied to the upper electrode 38 on and then off without performing X-ray irradiation. The data reader 66 then determines the amount of dark charge that occurs. The determined amount of dark charge is then subtracted from the total charge obtained after X-ray irradiation. The result is the net pixel charge produced by X-ray irradiation. That amount is then processed by the data reader 66.
In the conventional method described above a considerable amount of dark charge is included in the total charge obtained during data scanning. Since the maximum amount of total electric charge is limited, as the dark charge gets larger the range of treatable maximum pixel charge amount is reduced. Therefore, a picture contrast ratio that is related to the difference between the maximum and minimum pixel charges for each photo-sensing cell is reduced. Furthermore, in the conventional driving method a signal variation phenomenon is produced by the turn-off of the high voltage. The signal variation produces a distortion and an image quality deterioration. In addition, the conventional driving method has a significant drawback in that the turning-on and turning-off work of the high voltage must be performed at least twice. This extends the total data acquisition time required for imaging, something that is unfavorable when speed is important.
Accordingly, the present invention is directed to a method of driving X-ray imaging devices that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method of driving an X-ray imaging device that is capable of improving a contrast ratio of a picture as well as shortening imaging time.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of driving an X-ray imaging device according to an embodiment that is in accord with the principles of the present invention includes the step of applying a high voltage signal having the same level to the photo-sensing means of each photo-sensing cell during a charging interval of electric charges and during a discharging interval of the charged electric charges.
A method of driving an X-ray imaging device according to another embodiment of the present invention includes the steps of applying a high voltage signal to the photo-sensing means of each photo-sensing cell; storing electric charges in a charging capacitor provided at each photo-sensing cell while maintaining the high voltage signal at a fixed level; and then discharging the charged electric charges using an auxiliary circuit connected to each photo-sensing cell while maintaining the high voltage signal at the fixed level.
A method of driving an X-ray imaging device according to still another embodiment of the present invention includes the steps of applying a high voltage for coupling an electric field to each photo-sensing cell such that electric charges produced at a photo-sensing layer can be stored in a charging capacitor; reading dark charges stored in the charging capacitor of each photo-sensing cell using a data reader at an application time of the high voltage to remove them; irradiating an X-ray onto the photo-sensing layer; and reading pixel charges stored in the charging capacitor of each photo-sensing cell during X-ray irradiation using the data reader.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.